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Aviation History
1955
1955 - 0609.PDF
FLIGHT, 13 May 1955 609 MORE THOUGHTS on JET LIFT Getting Down To Economics IN a recent issue of Flight a description was published of a"jet-lift" aircraft of 300,000 lb all-up weight, with a wing load-ing of 300 lb per square foot, a thrust loading of 2.5 lb per lb, and provision for a maximum jet lift of 500,000 lb. Criticismscould be—and in fact were—levelled against this design; but my purpose here is not to attack or justify it, but to use it as abasis for a discussion of various possibilities in the application of jet lift to aircraft in general. The fundamental problem in any such design is to achieveacceptable take-off and landing performance at the minimum cost in terms of lift engines and fuel. In this particular case, take-off and landing were both vertical—the best possible performance; but it remains to be seen whether the achievement of this idealwas worth the cost. The total weight of the 50 lift engines amounted to 50,000 lb. The author gave no figures for pay-load,but whatever it might have been, it is evident that we could make a substantial addition to it if we were able to dispense withsome at least of the lift engines. There is also the question of the amount of fuel to be carried forlifting purposes; let us see how much this would be for the original specifica-tion. During the V.T.O., all lift engineswould be running, with a fuel consump- tion of slightly more than 200 lb persecond; if we allow no more than five seconds for this, it will still entail theuse of 1,000 lb of lift fuel. From this point the aircraft accelerates horizon-tally under the power of the main engines until it reaches the minimum safe wing-borne speed, which could hardly be less thanabout 240 knots; at the beginning of what we may con- veniently call the "take-off run" 20 of the lift engines wouldbe closed down, so that the jet lift just balanced the a.u.w., and the remainder would be cut out in succession, as the liftof the wing built up to a point where it was capable of support- ing the whole weight. As the lift of the wing varies as thesquare of the air speed this cutting-out process would not be uniform but would begin slowly and speed up towards the endof the run; thus the mean lift taken from the jets would not be half the maximum lift but about two-thirds, i.e., 200,000 lb. Thetake-off run would occupy about 35 seconds, and the lift fuel consumed would come to about 3,000 lb, giving a total consump-tion for the take-off of 4,000 lb. For landing the process would be reversed, but the "landing-run" would be considerably longerthan the take-off run, since the aircraft's drag would be the only retarding force; even if we allow 50 per cent reverse thrustfrom the main engines it is doubtful if we could bring the aircraft to rest in less than 60 seconds. If we take the landing weight as,say, 230,000 lb, the fuel consumed in the landing run would amount to about 3,800 lb. The actual let-down might occupyabout 15 seconds, during which time a further 1,400 lb would be burnt. Thus the total consumption of lift fuel per flight wouldbe not less than 9,000 lb, to which must be added at least another 9,000 lb reserve for emergencies. The weight of the whole jet-lift installation, then, is about 68,000 lb—say 70,000 lb includ- ing the weight of fuel tanks, etc. This means that if we removed the whole installation, and reliedupon a catapult, perhaps, or a pick-a-back aircraft to get the machine into the air, and parachutes to effect a safe landing,we could increase the pay-load by 70,000 lb (less the weight of the parachutes). The desirability of reducing the weight of liftjets and fuel to the absolute minimum hardly needs stressing therefore. An aircraft of this type could not conceivably be operated in(or within some miles of) a city or built-up area; hence V.T.O. is an unnecessary luxury. Let us throw out 20 of the 50 liftengines, thus saving 20,000 1b in tare weight (beside a large proportion of the first cost and maintenance costs) and see ifwe can make do with only 30. With 30 lift jets in operation the aircraft will not leave theground, but its effective weight will be reduced to nothing. If we stand it on the brow of a cliff, facing out to sea, it will moveover the edge as soon as the main engines are opened up; but it will not fall into the sea. Having no weight, it willcontinue to accelerate in a horizontal path throughout the WHEN we published "Jet Lift" in our issue of February 4th of this year we expected it to arouse more than usual interest. In the event, some readers even considered such developments as those envisaged by Rolls-Royce, Ltd., so advanced as to be impractic- able—at the present time, at least This contribution, from "Quidnunc"—a reader who wishes to remain anonymous—is neither wholly "for" nor "against," but suggests a number of intermediate possibilities. take-off run. In this way we can save the whole of the lift fuelburnt in the V.T.O., which with 100 per cent reserve amounts to 2,000 lb. The landing case is slightly altered, too, As there isnow no question of making a spot landing in a restricted space and the surroundings of the landing area are open country we cansafely make the landing run at a much lower height than before, with a correspondingly quicker let-down. We might well save1,000 lb (2,000 with reserve) here also. Of course, the aircraft would not in practice be restricted tothe tops of cliffs for its operation; any small plot of ground above the level of its surroundings would do. But we must be on ourguard against the temptation to try to climb during the take-off run. As soon as the nose is raised, the lift thrust and the weightwill combine to give a resultant force opposed to the thrust of the main engines, thus prolonging the take-off. A climb of5 degrees would increase the time—and the fuel burnt—by 33 per cent; 22 degrees would bring the aircraft to a standstill. It isfatal to try to fly with the jet lift inclined backwards. If the restriction of the aircraft toraised ground for its take-off were not acceptable, we could very easily getover the difficulty. We have only to tilt the thrust-line of the lift engines, say,5 degrees forwards from the vertical and give the aircraft a ground-angle of5 degrees, and it will take off and climb away at this angle as soon as the enginesare opened up. Being weightless it will accelerate just as rapidly as before;the only penalty for a climbing take-off run will be an increase in the lift fuelburnt proportionally to 1 — cos #, where 8 is the angle of climb. A 5-degree climb would clear a 50-foot screen in less than 200 yards,and would add 60 lb to the all-up weight. Fifteen degrees would entail a weight increase of 200 lb. Even a vertical climb could beachieved at a cost of no more than doubling the fuel burnt in a normal take-off, or an addition to the all-up weight of 6,000 lb;and as our modifications have saved 24,000 1b so far, we are still 18,000 lb better off than the original design. The question now arises, Supposing circumstances compelledus to adopt a fairly steep angle of take-off, could we save some of the weight penalty by levelling off after we had passed the obstaclein our way? We could. If we fit our lift engines with jet deflectors adjustable in flight we can level off at any stage of thetake-off run; we can in fact vary our angle of climb as we will so long as we ensure that the jet lift is never directed backwards.If three-quarters of the run is performed horizontally, we shall only incur one-quarter of whatever penalty is appropriate to ourinitial angle of climb. It is interesting to apply this to the V.T.O. case. The original design envisaged a vertical lift to no morethan 300 feet. Our modified design would reach this height in about seven seconds, or one-fifth of the take-off time, so that if welevelled off at this height, thus following the flight-path originally specified, we should incur a penalty of only 1,200 lb. That is tosay, we can perform exactly as specified, while carrying a pay- load 22,8001b greater than before. Alternatively, we can carry a pay-load 18,000 lb greater than specified and take it vertically up to about 7,000 feet. As, however, there is no practical advantage ineither alternative we need not pursue the point. The fitting of jet deflectors would allow another interestingpossibility which is worth considering, however. Suppose that instead of cutting out the lift engines progressively along thetake-off run we keep them all running but deflect them further and further forwards as the lift requirement falls off, relying uponthe vertical component of their thrust to keep us in equilibrium, while using the horizontal component to increase our accelerationalong the flight path. What effect would this have on our fuel consumption ? The lift thrust will remain constant throughout the take-off, ata figure of 300,000 fb, entailing a fuel consumption of 125 lb per second. But the acceleration, beginning at 0.4g, would rise,rapidly at first and then more slowly, to a maximum of 1.4g, and the time would be cut down to less than 15 seconds. Thetotal take-off consumption would be no more than 2,000 lb, thus effecting yet another saving of 2,000 lb in our all-up weight.The effect in the landing case would be even more marked. At a landing weight of 230,000 lb initial retardation would be about
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